Rheological Property
Paste viscosity modification involves applying mechanical energy to decrease resistance to flow during the stencil printing process. This thixotropic paste shear refers to the reversible reduction in viscosity when a solder paste is subjected to a constant force, such as the stroke of a squeegee across an aperture. The material returns to its original, more viscous state once the stress is removed, which prevents the deposited paste from slumping or losing its shape after the printer releases the stencil from the circuit board surface.
Proper control of this time-dependent thinning action ensures that the paste fills the aperture effectively while maintaining defined edges on the substrate.
Measurement Mechanism
Rotational viscometers assess the behavior by applying varying rates of strain to the mixture to determine the recovery speed of the substance. Technicians calculate the coefficient of thinning by comparing the viscosity at different stress intervals. High values indicate a material that flows readily under pressure but retains its structural integrity immediately after the application force ceases.
Consistent performance requires tight regulation of the ambient temperature because thermal fluctuations alter the internal structure of the binder and the resulting response to force.
Assembly Consequence
Inconsistent thinning behavior leads to bridging between pads or insufficient volume on fine pitch components during the transfer process. A paste that fails to recover viscosity quickly enough spreads beyond the intended footprint, causing short circuits that inspection systems detect during automated optical verification. Conversely, insufficient thinning during the squeegee stroke prevents the material from releasing cleanly from the stencil, leading to missed pads or incomplete joints.
Operators manage these variations by adjusting the print speed and pressure to match the specific thinning profile of the batch. Precise management of this phenomenon dictates the ultimate yield of the high-speed assembly line.
